Yeh the variation of announced & real life size kept me hesitant of going for the 28s. And as my new rims are 18mm inner (up from 16mm) there was too meny variables to take chances of having a too tight fit.
but have to say those 2017 schwalbe pro ones are super fast! set up tubeless of course :-)
If your tyres are 25mm and the rims is 18mm internally, then they must be around 27mm by now, which is actually good news, especially with tubeless making it feel more like a 28mm, even 30mm.
I have a little bit of paint scrap on my carbon bike as the 25mm was just about tight enough, so that clearance look quite comfortable IMHO.
Maybe get the GP4000sII that usually come up a little bigger in 25mm? (more like 27mm on wide rims).